Clinical applications of ventricular assist devices to support patients with end-stage heart disease, as a bridge to cardiac transplantation, or as an end stage therapeutic modality have become an accepted clinical practice in cardiovascular medicine. It is estimated that greater than 35,000 persons suffering from end stage cardiac failure are candidates for cardiac support therapy.
Ventricular assist devices may utilize a blood pump for imparting momentum to a patient's blood thereby driving the blood to a higher pressure. One example of a ventricular assist device is a Left Ventricular Assist Device (LVAD). The LVAD is inserted into the left ventricle of the patient's heart where oxygenated blood enters the LVAD through a blood inlet of the LVAD. The LVAD then imparts momentum to the blood. By connecting a blood outlet of the LVAD to the patient's aorta, pumped blood may reenter the patient's circulatory system.
Ventricular assist devices, such as the LVAD, have heretofore utilized mechanical positive displacement pumps and rotary pumps. Positive displacement pumps draw blood into a chamber by increasing the volume of the chamber. Such mechanical pumps are normally large and prone to mechanical wear. The human heart is a natural example of a positive displacement pump. A rotary pump forces blood by the spinning of an impeller within the pump. In general, the impeller of a rotary pump imparts momentum to the blood through the use of impeller blades or vanes which push the blood.
Rotary blood pumps may be either centrifugal or axial. In a typical centrifugal blood pump, blood enters the pump along the axis of rotation of the impeller and exits the pump tangentially. In a typical axial blood pump, blood enters the pump along its axis of rotation and exits the pump along the axis of rotation.
Traditionally, rotary blood pumps include a rotor consisting of a shaft and an impeller coupled to the shaft. Mechanical bearings are used to stabilize the rotor, both axially and radially, so the impeller could remain free to rotate smoothly while being constrained in the axial and radial directions. Mechanical bearings within the volume of blood have become a source of thrombosis. Moreover, as the use of mechanical bearings necessitated the protrusion of the shaft beyond the pumping chamber, a seal was required to prevent the escape of blood from the pumping chamber. This too became a source of thrombosis and sometimes hemolysis, as well as premature wear.
To minimize the risk of thrombosis and failed seals, sealless rotary blood pumps have been developed. For example, U.S. Pat. No. 5,695,471 to Wampler relates to a sealless rotary blood pump in which the rotor or impeller can be suspended within the pumping chamber by the use of magnetic or fluid forces.
Magnetic or fluid forces used to suspend the impeller within the pumping chamber could serve to stabilize the impeller, allowing for rotation while preventing excessive axial or radial movement. Wearless stabilization of an impeller can be achieved by magnetic bearings and hydrodynamic bearings.
Several forms of magnetic bearings have been developed. In one form, passive magnetic bearings in the form of permanent magnets can be embedded in both the rotor and the pump housing to provide magnetic coupling that may keep the impeller suspended in position within the pump casing.
Active magnetic bearings in the form of electromagnets can be used, for example, in or on the pump housing, magnetically to couple with and to drive the impeller. Power to the electromagnets may then be varied, as required, to adjust the magnetic field in response to displacement so that the impeller may be kept in position.
Because of the complexity of active magnetic bearings, rotary blood pumps have been developed that use passive magnetic bearings and hydrodynamic bearings to provide axial and radial constraint of the impeller in the pumping chamber. For example, U.S. Pat. No. 6,234,772, to Wampler et al. (the '772 patent), provides radial restoring forces and hydrodynamic bearings to constrain axial motion.
There remains a need for smaller and more efficient rotary blood pumps. In particular, there remains a need for wearless centrifugal pumps with hydrodynamic bearings and improved continuous fluid flow paths within the pump to further diminish the risks of hemolysis and thrombosis in the blood being pumped. By developing more sophisticated rotary blood pump impellers with hydrodynamic bearings and passive magnetic bearings, the physical size, performance and efficiency of the rotary blood pump may be improved to the point where consistent and reliable therapeutic support may be provided.